1. Field of the Invention
The present invention relates to a method for the immobilization of mediator molecules on surfaces of metallic or ceramic materials which are used for implants such as artificial joints or also microimplants, for example so-called stents, as well as implants produced according to the method.
2. Description of the Related Art
The implantation of artificial joints or bones has gained increasing importance in recent years, for example in the treatment of joint dysplasias or joint dislocations or in sicknesses resulting from joint attrition as a result of improper joint positioning. The function of the implants and the materials used for their production, which, in addition to metals such as titanium or metal alloys, can also include ceramics or synthetic materials such as teflon, have been continually improved, so that following a successful healing process, implants exhibit lifetimes of 10 years in 90-95% of all cases. Yet despite this progress and these improved operational methods, an implantation still remains a difficult and strenuous operation, particularly since it is associated with a long process of healing-in of the implants, often including month-long stays in clinics and health resorts, including rehabilitation measures. In addition to the pain, the length of the treatment period and the separation from familiar surroundings represent heavy stresses for the affected patients. In addition, the long healing process incurs high personal and treatment costs due to the required intensive care.
The understanding of the molecular-lever processes required for a successful growing-in of an implant has markedly increased in recent years. Structural compatibility and surface compatibility are crucial for the tissue tolerability of an implant. Biocompatibility in a narrower sense depends only on the surface. Proteins play a crucial role at all levels of integration. These form an initially adsorbed protein layer as early as during the implantation operation and thus, as explained below, since the first cells will later colonize on this layer, determine the further progression of the healing-in of the implant.
In the molecular interaction between implant, also referred to as biomaterial, and tissue, a multitude of reactions take place which seem to be strictly hierarchically ordered. The adsorption of proteins on the surface of the biomaterial is the first biological reaction which takes place. In the resulting protein layer, single protein molecules are for example either transformed by conformational changes to signal substances which are presented on the surface, or protein fragments functioning as signal substances are released by catalytic (proteolytic) reactions. Triggered by the signal substances, cellular colonization takes place in the next phase, and can include a multitude of cells such as leucocytes, macrophages, immunocytes and finally also tissue cells (fibroblasts, fibrocytes, osteoblasts, osteocytes). In this phase other signal substances, so-called mediators such as for example cytokines, chemokines, morphogens, tissue hormones and true hormones play a decisive role. In the case of biocompatibility, there is a final integration of the implant into the entire organism, and one ideally obtains a permanent implant.
In light of work performed in recent years at the molecular level of osteogenesis, chemical signal substances, the so-called xe2x80x9cbone morphogenic proteinsxe2x80x9d (BMP-1-BMP-13), which influence bone growth, have gained increasing importance. BMPs (in particular BMP-2 and BMP-4, BMP-5, BMP-6, BMP-7) are osteoinductive proteins which stimulate the formation of new bones and bone healing by effecting the proliferation and the differentiation of precursor cells to osteoblasts. Furthermore they promote the formation of hormone receptors, bone-specific substances such as collagen type 1, osteocalcin, osteopontin and finally mineralization. Here, the BMP-molecules regulate the three key reactions chemotaxis, mitosis and differentiation of the respective precursor cells. In addition, the BMPs play an important role in embryogenesis, organogenesis of bone and of other tissue, wherein osteoblasts, chondroblasts, myoblasts and vascular smooth muscle cells (proliferation inhibition by BMP-2) are known as target cells.
A particular aim in the immobilization method according to the invention is a degree of stimulation (that is, surface concentration of the immobilized protein) which allows a multivalent interaction between surface and cell and enables the effective control of bone and tissue formation.
To date, 13 BMPs including multiple isoforms are known. With the exception of BMP-1, the BMPs belong to the xe2x80x9ctransforming growth factor betaxe2x80x9d (TGF-xcex2) superfamily, for which specific receptors on the surface of the corresponding cells have been found. As the successful use of recombinant human BMP-2 and/or BMP-7 in experiments on defective healing processes in rats, dogs, rabbits and monkeys has shown, no species-specificity seems to exist. Previous attempts to exploit the bone formation-triggering characteristics of the BMPs for implantation purposes, in which BMP-2 and/or BMP-7 were noncovalently applied to metallic or ceramic biomaterials, have however been largely unsuccessful.
The goal of the present invention is to produce improved biomaterials for use as implants.
FIG. 1 is a photograph showing various substrates with oxidized TiO2 flakes.
FIG. 2 is a graph showing hysteresis measurements of various surfaces.
FIG. 3 is a graph showing the change in contact angle and hysteresis with non-oxidized titanium flakes following APS-modification and protein coupling.
FIG. 4 is a graph showing the change in contact angle and hysteresis with oxidized titanium flakes following APS-modification and protein coupling.
FIG. 5 is a graph showing the reduction of non-specific adsorption of fibrinogen by agarose coating of quartz glass plates.